The ultrasound sensors use the Doppler Effect and are volumetric motion detectors. They consist of the receivers, transmitters and processing electronics among other components and work by sending the sound signals that are of the high frequencies that are above human hearing and receiving the frequency that bounces back.
Technical Background about Ultrasonic Sensors
The sensors are devices that are used to determine the presence or absence of an object. They have transducers that send out signals and wait to receive back another signal.
An example of the transducers are the electrostatic transducers that can be termed to be similar to capacitors and have both a fixed plate made of Aluminum in most cases and a movable plate made of Kapton which is an insulator and is coated with gold. When a signal of 50Hz is applied the plate with the gold foil is attracted to the back plate.
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The other transducers that use the piezo effect are the Piezo electric transducers and they are used to measure the ultrasonic pulses. On the sensor is a ceramic material bonded to a cone or a metal case or they use a crystal. When a signal of 40Hz is applied, the crystal is excited and is attracted to the piezo material, causing it to expand or contract. The echo that returns causes the piezo material to vibrate thus generating a signal. These transducers are less costly than electrostatic transducers and better suited for the harsh environments.
In order to calculate the distance the following formula is used:
Distance = (elapsed time × speed of sound) ÷ 2
L= (vtcosθ) ÷2
Most of the limitations of the ultrasonic sensors are related to the Ohya because there is no way of determining what is in the path as any object in the signals path will return an echo. This can however be resolved by rotating a single sensor or using the multiple sensors. If multiple sensors are used, they can either be placed apart or at the same point but angled. This method gives a better idea of where the target is.
Difficulty in detecting the corners is another problem that the ultrasonic sensors have. An echo will be returned if one side of the opening falls on the side of the cone. To eliminate this problem, a narrowed cone can be used or the sensor can be placed closer to the openings.
Theory of the Ultrasonic Sensors’ Operation
Ultrasonic sensors are not biased to any color whether odd or loud. Even if the color changes no adjustments to the sensors are necessary as they continue operating as before and are also not irritated by the reflective objects. These sensors work independently from the surface’s appearance, no matter whether it is it smooth, retro reflective, reflective, symmetrical or asymmetrical in shape.
The environmental and weather conditions have little effect on the ultrasound sensors, thus making them very reliable under the harsh conditions. This is further supported by the fact that they are independent of the interferences. It is hard to deceive ultrasonic sensors as they will detect the material and make the required measurements as almost all materials reflect and affect the sound waves.
Ultrasound Sensors for Measuring the Level of the Underground Water Tanks
An underground water storage tank has about 10% of its combined volume above ground and may have an underground piping system connecting the tank.
Due to the effects of the global warming there has been a need to come up with the water conservation methods more so in arid and semi arid areas that are affected by the long periods of drought or in the other regions that experience unpredictable water shortages. One of the solutions to this problem is the use of the underground water storage tanks that can be made of concrete or plastic. In effect, this makes drinking water and water for the other domestic uses available and it is also a cost efficient method.
There is a need however to keep track of the amount of water that is in the tank at any specific time. If take for example, the water collected into the tank is from the rain, the easiest way to calculate the amount of water collected into the tank would be to multiply the size of the roof by the amount of rain that is received in that particular area regularly. A more accurate method of measurement is the use of the ultrasonic sensors.
The use of the ultrasonic waves was discovered by Lazzaro Spallanzani in 1790 after he realized that the bats used sound rather than sight to move. This later made steps forward when in 1826 Jean-Daniel Colladon using an underwater bell discovered sonography and he went ahead to determine the speed of the sound in water successfully. In 1881, Pierre Curies’ discovery set many enhancements in a motion for the modern ultrasound transducers’ creation. It was at this time that the interdependence of pressure on a crystalline material and electric voltage was discovered. The first ultrasound transducer was invented by Paul Langevin as a hydrophone that could send and receive the low frequency sound waves to detect the presence of icebergs after the Titanic mishap.
The ultrasonic sensors are placed on the ground above the tanks and transmit high pulse frequencies of 42 kHz. These signals travel from the sensor to the surface of the liquid and then back to the sensor as an echo. This sensor then measures the amount of time between when the signal was transmitted and when the echo was received back. A high accuracy of approximately ± 0.25% can be calculated depending on the exact distance between the sensor and the target. This helps to make the measurements of the underground water storage facilities.
The ultrasonic sensors are preferred in the measurement of the underground water tanks levels because the Electromagnetic Waves (EM) based devices are of the high speeds whereas the ultrasonic sensors are slower. It means that the time they take is much longer than that of EM waves. Their measurements are easier done and are also less expensive more so since they depend on the sound waves they work well in the wet environments.
Some of the advantages of the ultrasound sensors are such as: they can measure the distance between the source and the target, they can measure the stated distance both in short and in long range, there is no contact with the target object, they have a high resolution and a precise discrimination of the target location, they are independent of the surface texture of the target and its characteristics and are unaffected by the ambient light levels.
Some of the problems facing the ultrasonic sensing in detecting of the underground water levels are such as the fact that the speed of the sound is affected by the temperature changes of the medium. In this case, the ground temperature’ changes making the target seem like it has moved closer to the sensor even when the target is actually stationary. This will lead to the wrong made measurements.
The noise caused by the high pitched sounds such as the sound of the hissing pipes close to the input frequency that may interfere with the readings of the sensor thus generating readings that are not true of the conducted measurements.
The ultrasonic sensors also have a zone past which they cannot make the accurate measurements due to the ringing of the transducer when it continuously vibrates and emits a pulse.
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